Re: [PATCH 0/3] Allow user to request memory to be locked on page fault

On Thu, 14 May 2015, Michal Hocko wrote:

On Wed 13-05-15 11:00:36, Eric B Munson wrote:

quoted

On Mon, 11 May 2015, Eric B Munson wrote:

quoted

On Fri, 08 May 2015, Andrew Morton wrote:

quoted

On Fri,  8 May 2015 15:33:43 -0400 Eric B Munson [off-list ref] wrote:

quoted

mlock() allows a user to control page out of program memory, but this
comes at the cost of faulting in the entire mapping when it is
allocated.  For large mappings where the entire area is not necessary
this is not ideal.

This series introduces new flags for mmap() and mlockall() that allow a
user to specify that the covered are should not be paged out, but only
after the memory has been used the first time.

Please tell us much much more about the value of these changes: the use
cases, the behavioural improvements and performance results which the
patchset brings to those use cases, etc.

To illustrate the proposed use case I wrote a quick program that mmaps
a 5GB file which is filled with random data and accesses 150,000 pages
from that mapping.  Setup and processing were timed separately to
illustrate the differences between the three tested approaches.  the
setup portion is simply the call to mmap, the processing is the
accessing of the various locations in  that mapping.  The following
values are in milliseconds and are the averages of 20 runs each with a
call to echo 3 > /proc/sys/vm/drop_caches between each run.

The first mapping was made with MAP_PRIVATE | MAP_LOCKED as a baseline:
Startup average:    9476.506
Processing average: 3.573

The second mapping was simply MAP_PRIVATE but each page was passed to
mlock() before being read:
Startup average:    0.051
Processing average: 721.859

The final mapping was MAP_PRIVATE | MAP_LOCKONFAULT:
Startup average:    0.084
Processing average: 42.125

Michal's suggestion of changing protections and locking in a signal
handler was better than the locking as needed, but still significantly
more work required than the LOCKONFAULT case.

Startup average:    0.047
Processing average: 86.431

Have you played with batching? Has it helped? Anyway it is to be
expected that the overhead will be higher than a single mmap call. The
question is whether you can live with it because adding a new semantic
to mlock sounds trickier and MAP_LOCKED is tricky enough already...

The test code I have been using is a pathalogical test case that only
touches pages once and they are fairly far apart.

On the face batching sounds like a good idea, but I have a couple of
questions.  In order to batch fault in pages the seg fault handler needs
to know about the mapping in question.  Specifically it needs to know
where it ends so that it doesn't try and mprotect()/mlock() past the
end.  So now the program has to start tracking its maps in some globally
accessible structure and this sounds more like implementing memory
management in userspace.  How could this batching be implemented without
requiring the signal handler to know about mapping that is being
accessed?  Also, how much memory management policy is it reasonable to
expect user space to implement in these cases?

Eric